etheriwl: set msb for all rates

the 802.11 spec only specifies the msb of the rate for
Beacon, Probe Response, Association Response, Reassociation Response,
Mesh Peering Open, and Mesh Peering Confirm management frames
...
The MSB of each Supported Rate octet in other
management frame types is ignored by receiving STAs.

this should make no difference but on some netgear ap's not
setting this bit seems to ignore these data rates.

sys/src/9/pc/etheriwl.c

@@ -1818,15 +1818,15 @@ static uchar iwlrates[] = {
 	0x80 | 4,
 	0x80 | 11,
 	0x80 | 22,
-	12,
-	18,
-	24,
-	36,
-	48,
-	72,
-	96,
-	108,
-	120,
+	0x80 | 12,
+	0x80 | 18,
+	0x80 | 24,
+	0x80 | 36,
+	0x80 | 48,
+	0x80 | 72,
+	0x80 | 96,
+	0x80 | 108,
+	0x80 | 120,
 
 	0
 };